The invention is directed to a system of magnetic bearings for a high speed rotary vacuum pump with radially centering passive magnetic bearings, which originate axially destabilizing forces and thus an unstable point, with a dampening arrangement against radial vibrations, with emergency bearings and with electronic active axial position regulation with a position sensor, solenoid and pull-down disk fixed to form one piece with the rotor as a yoke for the solenoid, especially for a turbo-molecular pump.
Currently, most turbo-molecular pumps used are equipped with ball bearings, which are lubricated either with oil or grease. The disadvantage here is that during stoppage of the pump, for instance because of a power outage, lubricant vapors can penetrate to the high vacuum side and can contaminate the receiver or container and the components located therein. Pumps with magnetically supported bearings avoid this disadvantage.
A system of magnetic bearings regulated actively in five axes by solenoids is known, for instance, from DE PS 23 49 033. Also known are magnetic bearings for a turbo-molecular pump with a radially passive magnetic bearing regulated in three axes from DE-OS 33 41 716.
These sets of bearings are expensive because of the required technical resources for each regulating circuit with a sensor, electronics and solenoid per axis. Because of the availability of efficient magnetic compounds based on rare earth materials, there exists today the possibility of building a system of magnetic bearings with a single regulating circuit.
DE-PS 24 57 783 proposes a solution with an upper passive bearing acting axially and radially and an additional axially active bearing which is also effective passively radially. While in case of movements of the rotor in an axial direction, the force of the upper bearing increases according to the square law with increasing air gap widths, and the force of the lower bearing decreases at constant current in the electromagnet in accordance with the square law with decreasing air gap widths.
This poses considerable problems in linearizing the air gap changes measured by the position sensor and the required compensating forces which are a function of the square of the current and air gap. Furthermore, the centering radial force is proportional to the axial force and therefore not constant.
A system of emergency bearings for a set of magnetic bearings is known from DE-PS 25 24 061, which during normal operation, is kept at a distance from the shaft by an axially acting solenoid. In case of emergency bearing operation the lower stator of the emergency bearing is axially displaced by a spring, so that the shaft is centered in the emergency bearings.